The present invention, in some embodiments thereof, relates to photodetection and, more particularly, but not exclusively, to detection of long wavelength photons.
Recording and measuring a weak signal presents challenging and acute problems for the designers of modern sensors for myriad applications in diverse fields of science and technology. In these sensors, various primary signals (optical, ultrasonic, mechanical, chemical, radiation, etc.) are transformed into elementary charge carriers, such as electrons, holes or ions. Signal charge packets of such elementary charge carriers are amplified and converted to an electrical signal which is fed into a recording or analyzing device and/or used as a feedback signal for monitoring.
One approach to the detection of weak optical signals is the use of photodetectors in which the exposure times are long. These photodetectors typically employ semiconductor technology. Long exposure time photodetectors are suitable for static light source having constant intensity over time (e.g., stars), but are not suitable for rapid imaging applications in which the light has non constant emission intensity and/or originate from moving objects.
Another approach employs avalanche amplification (multiplication) of charge carriers. To date, avalanche amplification is recognized as a highly sensitive and high-speed method of amplification. Avalanche amplification is based on impact ionization arising in a strong electric field. The charge carriers accelerate in the electric field and ionize the atoms of the working medium of the amplifier, resulting in multiplication of the charge carriers. At a high multiplication factor, however, it is difficult to stabilize the avalanche amplification operating point. Additionally, the internal noise level and the response time grow rapidly with the multiplication factor.
Avalanche amplification based photodetectors are capable of converting a single photon to charge carriers and multiplying the charge. The number of photonic events is statistically estimated using the known quantum efficiency (QE) of the device. These photodetectors are suitable for static and well as dynamic light sources. Representative examples of such photodetectors include, high resolution arrays of photomultiplier tubes, avalanche photodiode array activated in the Geiger mode, electron multiplied CCDs, and intensified image sensors.
Avalanche photodiodes are the semiconductor analog to the photomultiplier tubes. By applying a high reverse bias voltage, an avalanche photo diode presents an internal current gain effect due to impact ionization. Unlike the photomultiplier tube, an array of the avalanche photodiode provides high resolution imaging with medium cost effectiveness. However, these devices suffer from high dark current and therefore require cooling to cryogenic temperatures for single photon imaging. The cooling requirement presents a major drawback to the technology because the cooling system significantly increases the power consumption, dimensions and cost of the device.
Since the energy of photon is inversely proportional to its wavelength the detection of long wavelength single photons, particularly in the infrared (IR) range, is more difficult.
IR detectors can be categorized according to the transport direction, the type of optical transitions, and the type of detection mechanism which can be photovoltaic or photoconductive. Broadly speaking, in response to light impinging on the detector, a photovoltaic detectors generates a measurable voltage (and current), while a photoconductive detector changes its conductance (or resistance).
Currently, prevalent infrared photodetection technology is based on interband (IB) absorption, wherein (IB) transitions occur in narrow bandgap semiconductors such as HeCdTe, InSb and InGaAs, mostly in PIN configuration. Another technology is based on intersubband (ISB) transitions in heterostructures in a configuration known as Quantum Well Infrared Photodetectors (QWIP), wherein the photodetection mechanism is via absorption between subbands rather than between the valence and conduction bands. An additional technology is based on type-II superlattice structures engineered by deposition of a stack of successive semiconductor layers. Superlattice detectors are also typically limited to cryogenic operation. Although much effort is invested in improving the performances of all types of IR detectors, none of the above technologies was proven to be sensitive enough for single photon detection.
Also known are devices called quantum dot field effect transistor (QDFET) in which Quantum Dots (QDs) are embedded in close proximity to a high mobility channel of a field effect transistor (FET) [A. J. Shields et al., APL 76, 3673 (2000)]. In this device, photoexcited carriers are trapped in the QDs and modify the channel conductance. It was shown that due to screening effect even a single photoexcited carrier can cause a measurable change in the channel conductance. This technology allows single photon detection at wavelengths of 900 nm and 340 nm for InAs and GaN QDs, respectively.